Three erratic boulders bearing cup marks and non figurative incisions were found. Two of the boulders were situated at Bòsc di Sàss (toponym derived from the great abundance of cobbles on the ground), at circa 330m a.s.l. in a wooded area not far from the Arno torrent. The first boulder has an elongated form (3 x 5.50m) with “double slope” with the longest side orientated on a NW-SE alignment. It is formed by a more consistent nucleus and by a slab-like part which has an artificial hollow in a central position. The incisions form a large linear-geometric image on the east side, which looks roughly like a cross flanked by a transverse groove. The second boulder is oval (1.20m x 0.60m) and stands at circa 25m NE of the first, on a slope facing towards the Arno torrent. It has two small cup marks (diam. 2cm, depth 0.5-0.8cm). The third boulder is situated at the boundary between the territory of Castronno (locality of S. Alessandro) with that of Sumirago (locality of Caidate), along a mule track which crosses a small water course nearby. It is sub-circular in shape, with the main axis on an E-W alignment (0.85 x 0.60m), and has a single cup mark (diam. 4cm; max. depth 1.5cm) The remains of a probable watch tower or military outpost were also investigated a few hundred metres away from boulders 1 and 2, near a dirt road which crosses the wood and is marked on 19th century land register maps. The remains stand on top of a hill in a thick chestnut wood: at least two structures were seen, of non definable shape, delimited externally by dry stone walls. (Carlo Liborio, Maria Giuseppina Ruggiero)